The present invention relates to magnetic devices such as saturable reactors, transformers, choke coils, accelerator cells, etc. for use in high-voltage pulse generating circuits used in pulse discharge gas lasers such as excimer lasers and copper vapor lasers, accelerators, etc.
One example of high-voltage pulse generating circuits for excimer lasers, one type of pulse discharge gas lasers, is shown in FIG. 27. The circuit of FIG. 27 is called a magnetic pulse compression circuit. DC voltage V.sub.i is applied between input terminals 201 and 202 in the polarity shown in the figure, and during the period in which a thyratron 204 is off, a main capacitor 206 is charged at a voltage V.sub.1 of about several tens kV in the polarity shown in the figure. In this circuit, a voltage V.sub.2 is applied between the terminals of a capacitor 207 after the thyratron 204 is turned on, and a saturable reactor 208 functions to compress the voltage V.sub.2 to a voltage V.sub.0 having a pulse width of about 100 ns necessary for the oscillation of the excimer laser. In this sense, this saturable reactor 208 may be called a magnetic switch. Incidentally, the pulse width of the voltage V.sub.2 applied to both terminals of the capacitor 207 depends on a time constant determined by capacitances of the capacitors 206 and 207 and an inductance of an inductor 205. 203, 212 denote inductances for charging the main capacitors 206, and 211 denotes electrodes for the discharge of the excimer laser.
In this circuit, since the pulse compression is achieved by using the saturable reactor 208, peak losses generated at the time of turn-on of the thyratron 204 and losses due to after current and inverse current can be suppressed, thereby contributing to high repetition rate, large output and long service life of the excimer laser.
FIG. 28 shows another example of high-voltage pulse generating circuits for excimer lasers, which is called a magnetic assist circuit. As in FIG. 27, DC voltage V.sub.i is applied between input terminals 221 and 222 in the polarity shown in the figure, and during the period in which a thyratron 224 is off, a main capacitor 226 is charged at a voltage V.sub.l of about several tens kV in the polarity shown in the figure. In this circuit, a saturable reactor 228 functions to delay the rise of the current i.sub.1, thereby decreasing switching losses generated at the time of turn-on of the thyratron 224. Likewise the circuit shown in FIG. 27, the circuit of FIG. 28 contributes to achieve the high repetition rate, large output and long service life of the excimer lasers.
As a further example of high-voltage pulse generating circuits, a circuit used in a linear induction accelerator, which is an accelerator of electron beam, etc., is shown in FIG. 29. As in FIG. 27, DC voltage V.sub.i is applied between input terminals 241 and 242 in the polarity shown in the figure, and during the period in which a thyratron 244 is off, a main capacitor 246 is charged at a voltage V.sub.1 of about several tens kV in the polarity shown in the figure. In this circuit, a transformer 253 functions to increase the voltage, and by setting the number of turns larger in a secondary winding 255 than in a primary winding 254, a voltage pulse having a larger wave height than that of the input voltage V.sub.i can be generated between both terminals of the secondary winding 255. Capacitors 247, 249 and saturable reactors 248, 256 constitute two steps of magnetic pulse compression circuits, which usually function to compress the voltage V.sub.2 having a pulse width of several .mu.m between the terminals of the capacitor 247 to a voltage V.sub.4 having a pulse width of about 100 ns or less between both terminals of a load 257. The load 257 is a conversion element called an accelerator cell for accelerating electron beams, etc. The accelerator cell functions like a kind of a transformer comprising a magnetic core. Incidentally, the details of high-voltage pulse generating apparatuses in a linear induction accelerator and the accelerator cells are shown in, for instance, D. L. Brix, S. A. Hawkins, S. E. Poor, L. L. Reginato and M. W. Smith, "A Multipurpose 5-MeV Linear Induction Accelerator," IEEE Conference Record of 1984, Power Modulator Symposium, pp. 186-190.
The magnetic device used for the above applications is usually a wound magnetic core composed of an amorphous magnetic ribbon and an insulation film or coating laminated alternately to have a breakdown voltage of about several tens kV or more. In the wound magnetic core, axial ends of the insulation film extend from those of the amorphous magnetic ribbon to prevent the insulation breakdown of the wound magnetic core due to discharge on the axial end surfaces. When it is used at a high repetition rate of several hundred Hz or more, the wound magnetic core is disposed such that it can be cooled by a coolant such as a compressed air, a freon gas, an insulating oil, etc.
FIG. 30 shows a saturable reactor capable of being operated at a high repetition rate, as one example of magnetic devices for high-voltage pulse generating apparatuses. In this figure, 1 denotes an input or output terminal, 2 a coaxial cylindrical conductor having an outer wall 2a and an inner wall 2b, 3 an output or input terminal, 4 an inlet for a coolant, 5 an outlet for a coolant, 6 a plurality of magnetic cores, 7 an insulating ring for fixing each magnetic core 6 to the inner or outer wall of the coaxial cylindrical conductor 2, and 11 an insulating seal member for providing insulation between the input and output terminals 1, 3 and for sealing a cavity defined by the inner and outer walls 2a, 2b of the coaxial cylindrical conductor 2. In this saturable reactor, the magnetic cores 6 are cooled by circulating a cooling oil by a pump (not shown).
FIG. 31 shows a transformer having a turn ratio of 1:1 as an example of transformers used in high-voltage pulse generating circuits. In this figure, 261 denotes a terminal common to primary and secondary windings of the transformer. One turn of the primary winding is constituted by the terminal 261, a cylindrical conductor 262, a rod conductor 263, a disc-shaped conductor 264, a rod conductor 265 and a primary winding end 266. On the other hand, one turn of the secondary winding is constituted by the terminal 261, the cylindrical conductor 262, a rod conductor 267, a disc-shaped conductor 268, a rod conductor 269 and a secondary winding end 270. Incidentally, a plurality of the magnetic cores 271 are fixed to the cylindrical conductor 262 by an insulating ring 272. In this transformer, the magnetic cores 271 are cooled by immersing the entire transformer in an oil bath.
FIG. 32 shows the structure of an accelerator cell used in the linear induction accelerator. An input winding having a turn number of 1 is constituted by terminals 281a, 281b, a coaxial cylindrical conductor 282 and terminals 283a, 283b, and an output winding having a turn number of 1 is constituted by terminals 291a, 291b, a coaxial cylindrical conductor 292 and a terminal 293. Incidentally, the terminals 283a and 283b and the terminals 291a and 291b are respectively connected electrically. A plurality of magnetic cores 286 are fixed to the coaxial cylindrical conductor 282 by insulating rings 287. The magnetic cores 286 are cooled by a cooling oil flowing from an inlet 296 to an outlet 297 in the direction shown by the arrow. 294 denotes a conical insulating seal member for sealing the high-voltage pulse generating circuit of the accelerator cell filled with an insulating oil from a space in which electron beams move.
In the above magnetic cores for high-voltage pulse generating apparatuses cooled by an insulating oil, heat spots tend to be generated inside the magnetic cores by magnetic core losses when a repetition rate is increased, for instance, to 1 kHz or more. As a result, the characteristics of the magnetic cores are deteriorated in a short period of time after starting the operation. In an extreme case, the magnetic properties of the magnetic cores at heat spots are drastically deteriorated, and their initial properties cannot be recovered after restart of the operation. Such deterioration of the magnetic properties due to the heat spots is remarkable particularly when the amorphous magnetic ribbon is used for constituting the magnetic cores.